Integrated circuits (IC's) are manufactured on semiconductor wafers using conventional photolithographic techniques. The process of manufacturing IC's involves many steps. One step includes depositing a photoresist layer on a semiconductor wafer surface. Photoresist layers are typically formed using a device called a photoresist coater. Photoresist coaters are well known in the art and typically spin coat photoresist material evenly onto the semiconductor wafer surface. The photoresist layer is subsequently exposed to light passing through a patterned reticule. Areas of the photoresist layer exposed to light are subsequently then removed using conventional development techniques to create a photoresist mask pattern. After formation of the photoresist mask pattern, a plasma etching operation can be applied to remove portions of the semiconductor layer exposed by the photoresist mask pattern.
The air surrounding the photoresist coater contains water molecules. As semiconductor wafers are loaded into the photoresist coater, dangling bonds of surface molecules (e.g., silicon, silicon dioxide, etc.) of the semiconductor wafer chemically attach to water molecules in the surrounding air. These attached water molecules may reduce the ability of the photoresist to adhere to the surface of the semiconductor wafer. Deposited photoresist often delaminates if the photoresist isn't sufficiently adhered to the surface of the semiconductor, particularly during the photoresist development step mentioned above. If the photoresist deposited on a semiconductor wafer delaminates, the semiconductor wafer must either be reworked or scrapped. Accordingly, it is important to remove all water molecules from the surface of the semiconductor wafer before photoresist is deposited thereon.
Semiconductor manufacturers often employ a photo resist adhesion promotion process to remove water molecules on the surface of semiconductor wafers before photoresist is deposited thereon. In this process, a semiconductor wafer is placed on a heating element (e.g., a hot plate) inside an airtight chamber of a device referred to as an adhesion promotion unit (APU). The heating unit heats the semiconductor to drive off unwanted water molecules (or other OH group molecules) attached to dangling bonds of surface molecules. A venturi vacuum device creates a vacuum which in turn removes air and other gases (e.g., water molecules driven off the surface of the semiconductor wafer) from the chamber as the semiconductor wafer is heated. Thereafter, hexamethyldisilizane (HMDS) vapor is introduced into the APU chamber. HMDS molecules attach to dangling bonds of wafer surface molecules so that the dangling bonds cannot later reattach to water molecules in the atmosphere when the wafer is removed from the APU chamber and moved to the photoresist coater. In essence, the adhesion promotion process results in a thin layer of HMDS on the surface of the semiconductor layer which in turn promotes adhesion of photoresist material.
If the APU is functioning correctly, the venturi vacuum device in conjunction with the heating element will drive off water molecules from the semiconductor wafer. APUs are equipped with pressure gages in fluid communication with the APU chamber. The pressure gages are visually checked once or twice a day by an operator to insure that a vacuum is created during the adhesion promotion process. APU components, unfortunately, are prone to undetected malfunctions that can prevent the creation of a vacuum which in turn prevents the removal of unwanted water molecules from the chamber. For example, an air leak may develop in the seal that makes the APU chamber airtight or in a conduit coupling the AiPU chamber and the venturi vacuum device, or the venturi vacuum device may fail. Any of these malfunctions or others may occur between visual checks of the APU pressure gage. If a sufficient vacuum is lacking within the chamber at the scheduled time the operator checks, all wafers previously processed since the last vacuum level check may be adversely affected and improperly processed prior to photoresist deposition. In other words, numerous wafers may have been processed with ineffective adhesion promotion before the condition of insufficient vacuum is discovered. Unfortunately, those improperly processed wafers will either require rework, or will be scrapped, depending on how far they have progressed before the low vacuum condition is discovered.